Réf. Stoffel 2007 - A

Référence bibliographique complète
STOFFEL M. Debris-flow activity in the Ritigraben torrent (Valais Alps, Switzerland): Will there be less but bigger events in a future greenhouse climate? Landslides and Climate Change / ed. by McInnes, Jakeways, Fairbank, Mathie, 2007, p. 51-58.

Abstract: Tree-ring records and climate proxies suggest that cool summers with frequent snowfalls regularly prevented the release of debris flows at Ritigraben between the 1570s and 1860s, whereas the warming trend in conjunction with greater precipitation totals in summers and falls apparently led to an increase in activity between 1864 and 1895 and in the early 20th century. Simultaneously, the seasonality of events started to shift from June and July to August and September. Given that RCMs project extreme precipitation events to occur less frequently in summer and that wet spells will become more common in spring or fall, it is conceivable that debris flows will not necessarily occur as frequently in the future as they did in the past. But even if the frequency of events is likely to decrease, the magnitude of summertime debris flows and related impacts could be greater than currently.

Mots-clés
Dendrochronology, Little Ice Age, debris flows, RCM simulations

Organismes / Contact
Laboratory of Dendrogeomorphology (Dendrolab.ch), Department of Geosciences, Geography, University of Fribourg, Switzerland.

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Precipitation   Torrential events Debris flow

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Swiss Alps Valais Ritigraben torrent, 46° 11’ N, 7° 49’ E   2600-3214 m
1500-1800 m
1570-2005

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations

 

Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

(3) - Effets du changement climatique sur l'aléa
Reconstitutions

Based on geomorphological reconstructions and seasonality analysis from meteorological records, it appears that snowfalls and frozen ground apparently inhibit debris entrainment from the starting zone (>2,600 m a.s.l.) during precipitation events between October and May.
It is possible to characterize climatological as well as meteorological factors driving debris-flow activity in the case-study area. Tree-ring based records of past debris-flow activity suggest that comparably cool summers with frequent snowfalls at higher elevations regularly prevented the release of debris flows most of the time between the 1570s and 1860s. The warming trend in conjunction with greater precipitation totals in summers and falls between 1864 and 1895 did, in contrast, lead to an increase of meteorological conditions favourable for the release of debris flows from the departure zone. Enhanced debris flow activity continued well into the 20th century and the reconstruction exhibits a clustering of events for the period 1916–1935, when warm-wet conditions prevailed during summers in the Swiss Alps (Pfister 1999).

Observations

 

Modélisations
 
Hypothèses

Debris availability and recharge rates could undergo changes in the departure zone due to changes in climatic conditions. Given that temperatures will rise by several degrees in a future greenhouse climate (according to e.g., the IPCC A2 scenario, see Nakícenovíc et al. 2000), it is conceivable that this will have consequences on the currently prevalent permafrost as well as on the dynamics of the active rock glacier that nowadays feeds the starting zone of debris flows with material. Preliminary results from borehole temperature measurements realised next to the departure zone of the Ritigraben torrent suggest that permafrost is comparably temperate and possibly in an unstable state (Herz et al. 2003). At other locations in the Swiss Alps (Roer et al. 2005, Delaloye pers. comm.), important accelerations have been observed in rock glacier movements (“surges”) over the last few decades. It is, thus, possible that rock glacier movements could increase at our study site in the future as well and therefore deliver more debris to the starting zone of debris flows in the Ritigraben torrent. In conjunction with more precipitation events >60 mm/day, these larger amounts of debris could theoretically lead to the entrainment of more material and subsequently to larger debris flows in the system.

On the other hand, should the ice once completely disappear from the rock-glacier body at Ritigraben, one could also imagine that debris would less easily be transported to the starting zone of debris flows and therefore less material be available for the initiation of future events.


Paramètres de l'aléa
Sensibilité du paramètre de l'aléa à des paramètres climatiques et du milieu / Facteurs de contrôle
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

 

See Stoffel & Beniston 2006 - A


(4) - Remarques générales

This stuy is a complement of Stoffel & Beniston 2006 - A, only complementary informations have been mentioned here.


(5) - Syntèses et préconisations

The above considerations on potential changes in the seasonality of heavy precipitation events and on potential modifications of rock-glacier dynamics remain highly speculative for the moment. Nonetheless, local authorities should not wait with the planning of appropriate constructive measures so as to (better) protect the buildings located along the currently used channel as well as on the intermediate debris-flow cone from future damage. Regardless of expected changes in the frequency or magnitude of debris-flow events at Ritigraben, they have to be aware that considerably large debris-flow events have repeatedly occurred in the past, and that they will occur in the future as well. As the lateral walls of the currently used channel (that have been incised by the September 1993 event) already started to collapse, it is possible that future debris flows could overtop the channel above 1,650 m a.s.l., reactivate abandoned flow paths and deposit material in the eastern or south-western parts of the cone, theoretically threatening buildings and public infrastructure.